K.D.
My son did the same thing. From when he was about 2 1/2 and gradutated to his big boy bed till about 4! The difference between us though was I was either too lazy or too tired to take him back to his bed so he just ended up sleeping with us. I am pregnant now with #3 and decided that just isn't going to work so I NEEDED to get him out of this habit. I created a sticker chart for him and put it on the refrigerator. For every night he slept in his bed ALL night he would get a sticker; after 20 stickers (20 nights sleeping in his own bed) I would get him a small toy. Then again for 20 nights ( a medium toy) and and one more time for a big toy and hopefully, after the 60 nights he would be broke of the habit. It only took about 20 nights total for him to receive 13 stickers and he now sleeps in his own bed all night. Even nights when he doesn't feel good or I ask him if he wants to sleep with me, he says "NO, I won't get a sticker!" I worked MUCH better then I ever planned!
Good Luck!!